Used water-in-oil drilling muds, as they are returned from the well, are virtually unfit for reuse without rejuvenation. They contain drill cuttings and other solids, which can be removed in a known manner, but the most difficult problem with the remaining fluid is that it contains too much water. Drilling fluids are commonly in the form of water-in-oil emulsions, typically having an original ratio of 70-90% oil to 10-30% water (by volume), apart from the solids content. The oil/water ratio influences the rheology of the fluid and is an important factor in maintaining the drilling mud properties. For example, when a 90/10 ratio emulsion returns to the surface after use, it might have been diluted to perhaps 70/30 v/v in addition to entraining significant amounts of solids such as drill cuttings. The used mud will also commonly contain other materials picked up from the formation and may have changed chemically and physically in other respects; in addition it is likely to have reduced contents of one or more beneficial additives, but the major difficulty is that, to restore its usefulness, the ratio of oil to water must be restored to the desired figure. One way to do this is to add more oil and other ingredients, which requires more storage and increases the cost of the fluid by the amount of excess oil and ingredients required. In some cases it takes literally two times the original volume to correct the oil/water ratio.
For example, if the original volume/volume ratio was 9.0 oil/1.0 water (10% water by volume), and the returned material is 8.0 oil/2.0 water (20% water), restoring it to the desired ratio of 9:1 by adding oil would require ten (10) volumes of oil, doubling the overall quantity and creating problems in both mixing and storing. Ratios lower than 8:2 are common in the returned fluid. The problem is even more severe for a ratio of, for example, 7:3—to restore this mixture to 9:1 with oil alone would require about 3.85 volumes of oil.
Both environmentally and economically, it is very desirable to recycle used drilling mud. Doing so greatly reduces the need for water and additional drilling chemicals, but also drastically alleviates the waste disposal problems of the operators.
The desired ratio of oil to water can be restored by removing excess water rather than adding oil and the other drilling fluid ingredients. The original emulsions, however, were carefully constructed using excellent emulsifiers, and are difficult or expensive, or both, to break by chemical means or by simple boiling even when additional water has upset the original emulsification balance. On the other hand, used drilling fluids have been subjected to great stress, and the original even dispersion is seldom preserved—that is, the discontinuous phase will have been disrupted and reconstituted by those excellent emulsifiers, now having additional water to work with, to a range of sizes not conducive to good purpose for reuse in a well. The process of reducing the high water content of a used mud should include, if possible, an even redistribution of the discontinuous phase in the desired dimensions. Another common characteristic of drilling fluids is that they contain significant amounts of calcium chloride, deliberately added to “hold” the water in the emulsion as it is subjected to turbulence and stress. The presence of the calcium chloride makes the task of removing water from the emulsion more difficult. A practical method of restoring the original ratio should conserve time and expense. The ability to process the used mud on return to the surface would help to reduce storage of the mud and inventory of various ingredients. A practical method of removing water from water-in-oil emulsions, particularly one which can lead to ready-to-use drilling muds, is needed.